Many water absorbent materials including natural materials (e.g., sponge, pulp and paper) and synthetic materials (which may be prepared by partially neutralizing a hydrous polymer having a hydrophilic group, e.g., --OH, --NH.sub.2 or --COOH, with a salt) have been used conventionally as sanitary materials and in the agricultural field. However, since they absorb water or other fluids through a physical process, they tend to have a low water absorption capacity and, consequently, they have a limited ability to retain the water absorbed especially when pressure is applied.
To improve the absorption capacity, therefore, various synthetic absorbent materials which can attract water through both physical and chemical mechanisms have been developed; and, representative examples thereof include a partially crosslinked polyacrylic salt, saponificated starch-acrylonitrile graft copolymer, crosslinked cellulose-acrylic acid salt graft copolymer, etc. They are useful as sanitary materials such as diapers and sanitary napkins, and in a civil engineering and gardening field. They are also useful in other fields such as coagulation of sludges, dehydration of oils and prevention of dew condensation in contruction materials.
Hithereto, many such improved water absorbent resins useful as sanitary materials have been developed. However, in the sanitary materials field, particulary disposable diapers and menstrual napkins, it is very important for the water absorbent resin employed to have a good suction power and gel strength.
The suction power as used herein means the force of sucking water from pulp that has absorbed water; and the gel strength means the stability of the fluid-absorbed state.
U.S. Pat. No. 4,497,930 discloses a method for preparing an absorbent polymer, which comprises: polymerizing reaction monomers; dehydrating the resultant hydrophilic polymer to provide a water content ranging from 10 to 40% by weight based on the total amount of the hydrophilic polymer; and crosslinking said hydrophilic polymer in the presence of a crosslinking agent having at least two functional groups in the molecule, which are capable of reacting with carboxyl or carboxylate groups. However, in this method, since the crosslinking agent is added together with water in an amount that can barely dissolve the crosslinking agent therein, its presence tends to be limited on the surface of the polymer. Therefore, although the water absorption capacity and absorption rate of the polymer may be good, the suction power and gel strength tend to be low.
U.S. Pat. No. 4,541,871 discloses also a method for preparing an absorbent resin, which comprises: crosslinking a resin having a carboxylate group with 0.005-5% by weight of a crosslinking agent, in the presence of water in a proportion ranging from 0.01 to 1.3 parts by weight per part by weight of the resin. In this method, since water is removed in an amount of less than 0.5 parts by weight on the basis of the added amount of water during the dehydration process, the resultant resin has a poor suction power and gel strength.
As described in the above, the absorbent resins prepared by the methods described in the above-mentioned patents may have good water absorption capacity and absorption rate, but have poor suction power and gel strength to be useful for sanitary materials.